For denim, use a jeans (denim) needle sized to the fabric: 90/14 for light denim, 100/16 for midweight and 110/18 for heavy denim over about 12 oz/yd² (400 GSM). Sew the seams with all-purpose polyester thread at 2.5–3 mm, topstitch with a heavier topstitching thread and a longer 3–3.5 mm stitch, finish seams with flat-felled or overlocked edges, and level the presser foot over thick seams with a folded scrap or a hump jumper.
How heavy is your denim?
Denim weight is usually given in ounces per square yard (oz/yd²). Multiply by 33.9 to get grams per square meter, or GSM, the unit most European shops use.
| Weight | oz/yd² | GSM | Typical use |
|---|---|---|---|
| Light | 4–7 | 135–235 | Shirts, dresses, gathered skirts, shorts |
| Mid | 7–12 | 235–405 | A-line skirts, jeans, jackets, bucket hats |
| Heavy | 12–16 | 405–540 | Workwear, bags, heavy jeans |
Stretch denim contains 1–3% elastane. It is more comfortable in fitted garments but can grow at the knees and seat, so keep it for fitted styles and choose rigid denim for skirts, bags and structured pieces.
Which needle and thread should you use for denim?
| Task | Needle | Thread | Stitch length |
|---|---|---|---|
| Seams in light denim | Jeans 90/14 | All-purpose polyester | 2.5 mm |
| Seams in mid denim | Jeans 100/16 | All-purpose polyester | 2.5–3 mm |
| Seams in heavy denim | Jeans 110/18 | Heavy-duty polyester | 3 mm |
| Topstitching | Topstitch 90/14 or 100/16 | Topstitching thread (around 30 wt) | 3–3.5 mm |
A jeans needle has a stiff shaft and a sharp point that goes through tightly woven twill without bending. A topstitch needle has a long eye and a deep groove so thick thread passes freely. Keep all-purpose thread in the bobbin when you topstitch: heavy thread in the bobbin rarely works on a home machine. The needles and thread guide covers the full range.
If the topstitching looks loose underneath, raise the top tension a step or two. Always test on a scrap folded to the same number of layers as the real seam, because tension that suits two layers often fails at six.
Can a home sewing machine sew denim?
Yes, within reason. Most home machines handle four to six layers of midweight denim if you fit the right needle and go slowly. The trouble starts at bulky seam junctions, where the presser foot tilts up, the stitches shrink and the needle can stall.
- Level the foot. As you approach a thick seam, slide a folded scrap of denim or a hump jumper (a small plastic shim) under the back of the foot so it climbs onto the bump level.
- Turn the hand wheel through the thickest part instead of pressing the pedal.
- Flatten the seam first. Press it with steam, then tap the bulky area with a mallet through a cloth to compress the fibers.
- Grade and trim. Where many layers meet, trim the seam allowances to different widths so the bulk steps down.
If the machine stalls repeatedly or a needle breaks, stop. Jeans hems with eight or more layers are hard work for lightweight machines, and forcing them can knock the timing out.
Which seams work best for denim?
The classic denim seam is the flat-felled seam: one seam allowance is trimmed, the other folded over it and stitched down, giving two rows of topstitching on the outside and no raw edges inside. It is strong, flat and survives countless washes.
Without a felling foot, a mock flat-felled seam is easier: sew a normal seam, overlock or zigzag the two allowances together, press them to one side and topstitch twice from the right side, about 6 mm (1/4 in) apart. From the outside it looks almost identical.
For skirts and bags, a plain seam with its allowances finished by overlock or zigzag and pressed open is enough.
Do you need to pre-wash denim?
Yes, unless you are deliberately working with raw denim and planning for its shrinkage. Sanforized (pre-shrunk) denim still shrinks a little, often 1–3%; unsanforized raw denim can shrink much more. Indigo also bleeds, so wash new denim inside out, cold, on its own or with dark colors, for the first few washes. The pre-washing guide has the general method.
How do you cut denim?
Denim's twill lines run diagonally, and its shade changes slightly with direction, so cut every piece with its top pointing the same way along the length, as you would a fabric with a nap. Use sharp shears or a rotary cutter with a fresh blade, and mark with chalk, which shows well on indigo.
Snip notches into the seam allowance (2–3 mm deep) rather than cutting them outward: outward notches are hard to cut cleanly through firm twill, and snips are easier to find on a dark fabric.
How do you hem denim neatly?
A double-fold hem is the denim classic: fold up 1.5 cm (5/8 in), then the same again, press hard with steam and topstitch close to the inner fold. On midweight denim that is six layers at every seam, so this is where the hump jumper and the hand wheel earn their place.
To reduce bulk, trim the seam allowances inside the hem to half their width before folding, or press the seams open within the hem area. Sew the hem from the right side with topstitching thread if you want the visible stitch line, and finish the thread ends by pulling them to the wrong side and knotting them, since backstitching with thick thread looks heavy.
What can you sew with denim?
Denim suits structured shapes that hold their form. Good projects for a home machine include an A-line skirt in midweight denim, a tote bag with a denim outside and a cotton lining, a reversible bucket hat, a work apron, and pull-on shorts in light denim or chambray. Old jeans are a good source for the smaller pieces.
Sewly does not yet draft jeans with a fly front. For a first denim garment, an A-line skirt teaches the same skills, from topstitching and thick seams to a zip and a waistband, with far fewer layers to fight through.